Forty patients with hypereosinophilic syndrome (HES), defined according to the criteria established by Chusid et al., were studied retrospectively in order to determine the prognostic factors in this heterogeneous disease. A myeloproliferative syndrome (MPS) was diagnosed 17 times when splenomegaly and/or a greater than 5 times the normal vitamin B12, level existed. Cutaneous-visceral involvement was almost always present (95%), but could appear after several years of evolution. Twenty-three patients had cardiac involvement, determined by clinical, radiological, electrocardiographic and echocardiographic examinations; 12 of these had endomyocardiac fibrosis. The overall survival rate was 80% at 5 years and 42% at 10 and 15 years. Among the factors evaluated that are capable of influencing this survival, 5 were found to impair the prognosis; presence of an MPS; non-response of the hypereosinophilia to corticoids; existence of a cardiopathy; being male; and an elevated maximum eosinophilia; the last two factors were significantly associated with the cardiopathy. Because hematological or cardiac anomalies are not always present at the time of the first examination, HES patients require very close surveillance.